I. Field of the Invention
This invention relates generally to electronic apparatus, and more particularly to a system for simulating the transit time of a transmitted radar signal.
II. Discussion of the Prior Art
As those skilled in the art will appreciate, a radar altimeter for an aircraft periodically sends out a transmitted pulse and, a short time later, an echo pulse, returned from the ground, triggers a radar receiver in the aircraft. By knowing the delay time between the generation of the transmitted pulse and the receipt of the received echo pulse, the distance between the aircraft and the earth's surface can be inferred. In bench testing such a device, it is desirable to be able to simulate the transmitted pulse's time so that the altimeter may be properly calibrated.
In prior art radar test apparatus, it has been the practice to utilize a plurality of lumped constant delay lines, each with its own individual delay characteristic. These discrete devices are operatively disposed between the radar system's transmitter output and its receiver input. In this regard, reference is made to the Bush et al U.S. Pat. No. 4,121,213 which, in FIG. 1 thereof, depicts the aforementioned prior art design. Because precision calibration of radar equipment, such as radar altimeters, would require that the delay lines be carefully tailored, this adds significantly to their cost. Then, too, it is necessary to maintain an inventory of spare parts for each of the delay elements. Such fixed length acoustical delay line elements currently cost between $3,000 and $5,000 each and because an inventory of spares is usually required for each test device, a major portion of the material cost for the system is due to the number of delay lines required. This cost factor tends to limit the number of simulated altitudes that can be provided by the test equipment.